1. Field of the Invention
The present invention relates to an ink jet recording head that ejects ink, and an ink jet recording apparatus that records an image using the ink jet recording head.
2. Description of the Related Art
A bidirectional recording method is known as one of recording methods in an ink jet recording apparatus. In a serial type recording apparatus, the recording head first performs scanning in a forward direction for recording, and then paper is fed by a predetermined amount. Next, recording scanning is also performed during backward movement of the recording head. This recording method achieves an approximately double recording speed or throughput compared to unidirectional recording in which recording is performed during forward scanning, whereas recording is not performed during backward return movement of the recording head.
The bidirectional recording method is effective means in improving the recording speed or the like as described above. However, it is known that color varies among scanning areas and this leads to color nonuniformity or color misregistration in the whole image. This is because the application order of respective colors of inks differs between the forward and backward directions of the bidirectional recording. In a recording apparatus, ejection orifice rows (nozzle rows) for respective colors of inks are commonly arranged in the scanning direction. However, the application order of inks is reversed between the forward scanning and the backward scanning depending on the arrangement of the ejection orifice rows.
When dots of a predetermined color are formed by applying (ejecting) a plurality of kinds of inks so that these inks overlap on a pixel, inks applied to the recording medium earlier develop their colors more strongly. This is because the inks applied to the recording medium earlier easily color the material in a layer closer to the surface of the recording medium, while the inks applied to the recording medium later less easily color the material in the surface of the recording medium and permeate deeper through the recording medium in its thickness direction before they are fixed. This phenomenon is significant when coated paper is used that has a silica layer as an ink receiving layer. However, this phenomenon also occurs on plane paper or a gloss recording medium that has a gloss layer formed in the surface thereof and an ink receiving layer formed in the inside thereof.
Japanese Patent Laid-Open No. 2000-318189 describes a configuration that eliminates color nonuniformity or the like attributed to the application order of inks. In this configuration, two nozzle rows are provided for each color of ink and arranged symmetrically with respect to an axis perpendicular to the scanning direction.
FIG. 1 shows the configuration of a color ink chip 1100 of a recording head in Japanese Patent Laid-Open No. 2000-318189. In the figure, a pair of nozzle rows c1 and c2 that eject cyan ink, a pair of nozzle rows m1 and m2 that eject magenta ink, and a pair of nozzle rows y1 and y2 that eject yellow ink are each arranged symmetrically with respect to an axis perpendicular to the scanning direction X. When recording is performed using this recording head by the bidirectional recording method, in order to form ink dots in each pixel, the inks are ejected (applied) in the order of c1, m1, y1, y2, m2, and c2 in the forward scanning. On the other hand, in the backward scanning, the inks are ejected in the order of c2, m2, y2, y1, m1, and c1. In this way, the inks can be applied or overlap one another in the same order between the forward scanning and the backward scanning. As a result, regardless of the scanning direction of the recording head, the inks can be applied in the same order, and therefore color nonuniformity attributed to bidirectional recording can be reduced.
On the other hand, black ink nozzle rows k1 and k2 are also provided in the chip 1100. The positional relationship between the nozzle rows k1 and k2 and the other ink nozzle rows is the order of k1, k2, c1, m1, y1, y2, m2, and c2. In this case, the order in which black ink and other inks overlap varies depending on the scanning direction. However, when dots of an image to be recorded are formed using only black ink, the above overlapping with other inks does not occur.
However, in reality, when a high density part is expressed, black ink is often used rather than using chromatic dye inks such as cyan, magenta, and yellow inks. This is because, compared to recording a black image in black (process black) expressed using cyan, magenta, and yellow dye inks, forming a black image using black ink uses a smaller amount of ink and therefore can prevent problems such as ink overflow. In addition, when black ink is used, the optical reflection density of the recorded black image is high, and high contrast recording can be achieved.
However, when a relatively low density part is expressed in black ink, the number of ink dots shot per unit area is small, and therefore the area on the recording material where ink is not shot is large, and the color of the recording paper can be seen. Many widely-used recording papers (recording media) are light in color, for example, white. Therefore, compared to cyan, magenta, and yellow inks, black ink has low lightness, and the difference in lightness from the recording medium is large. Therefore, the shape of each recorded ink droplet can be easily visually noticed. As a result, disadvantageously, the recorded image appears rough, and the smoothness decreases.
Therefore, hitherto, by using process black for recording a relatively low density part, less granular and smooth gradation expression has been achieved.
On the other hand, in a relatively high density part, black ink and color inks are superposed. However, process black ink and black ink are rarely superposed, and therefore color nonuniformity due to bidirectional recording-induced difference in the ink application order occurs rarely.
Recently, there has been known a recording apparatus that uses gray ink for the purpose of further improvement of image quality. According to this recording apparatus, the dot shape of gray ink, which is lighter than black ink, on a recording medium is less noticeable than that of black ink, and so granular impression can be reduced. Therefore, a recording apparatus that uses achromatic black and gray inks and chromatic cyan, magenta, and yellow inks uses gray ink and cyan, magenta, and yellow inks in a relatively low density part.
As described above, some ink jet recording apparatuses form dots by superposing gray ink, which is achromatic ink, and chromatic inks such as cyan, magenta, and yellow inks. In this case, depending on the order in which a nozzle row for gray ink and nozzle rows for other colors of inks are arranged in the sub-scanning direction, the order in which inks are applied or superposed varies and color nonuniformity can occur. To solve this problem, it is possible to provide two nozzle rows for gray ink as with other color inks and to dispose the two nozzle rows symmetrically with respect to an axis perpendicular to the scanning direction.
However, when two nozzle rows are provided for one color, the relative positional relationship between nozzle rows is liable to deviate from the ideal positional relationship. For example, attaching errors of nozzles can occur in the process of producing recording heads. Even if a recording head is free from manufacturing errors, the recording head can be obliquely mounted in a main body of a recording apparatus. In this case, landing positions of ink droplets ejected from nozzle rows onto a recording medium deviate from ideal positions, and change of color shade, streaks, density nonuniformity, and so forth occur in the recoded image. This leads to image deterioration.